The properties of cholesterol and cholesterol esters in aqueous solution are being investigated together with the interaction between these two compounds and phospholipids and phospholipid-protein complexes. The apo-proteins of high density (HDL) and low density (LDL) serum lipoproteins are being probed for hydrophobic binding sites using a number of hydrophobic ligands such as alkanes and alkyl sulfates. In addition, the association between the 28,500 polypeptide chain and the 17,400 disulfide bonded dimer of HDL is being studied as a function of ligand binding. These investigations are expected to provide structural and thermodynamic data relating to the interactions of cholesterol, phospholipid, and protein in the serum lipoproteins.